As a method of dividing a brittle material, for example, a large glass plate, into a smaller size, a method may be used involving introducing a scribe line (notch) on the surface along an intended cut line (a line along which glass will be cut) with a superhard tool blade such as tungsten carbide and polycrystalline diamond.
In this method, a glass plate is bend-broken in a mechanical manner by applying bending stress in a direction orthogonal to the scribe line (bend-breaking).
However, in the mechanical cutting approach as described above, glass plates may be brought into contact with each other upon bend-breaking to produce small fragmented pieces called “cuttings.” This may contaminate the surfaces and cut surfaces of cut glass plates. Further, countless minute notches called “microcracks” may be created on the cut surfaces due to bend-breaking, resulting in a decreased mechanical strength of the cut glass plates.
Further, uniform application of bending stress over an intended cut line is difficult for a thick glass plate which is mainly used in buildings and the like (for example, a glass plate with a plate thickness of 15 mm or more) upon bend-breaking, often resulting in cut-end defects described in JIS R3202 such as “splinters,” “horned projection,” “chipped cut-end,” “shell-like chipping,” “deviation from cut line.”
For these reasons, the thermal stress cutting process has also been used in recent years, including the full-body cutting process (which means that a material is divided into two. “Divided into two” means that a material is divided into two fragments with respect to one intended cut line.) by laser beam irradiation.
It is said that the laser beam irradiation can avoid defects characteristic of the conventional mechanical cutting process such as contaminated cut glass plates due to production of “cuttings” upon cutting and decreased glass strength due to development of “microcracks,” and thus can generally provide cut surfaces with higher strength as compared with those by the mechanical cutting process.
Patent Document 1 describes a technology for full-body cutting of a brittle material by using wavelength-tunable laser. According to the above technology, a laser beam is allowed to be absorbed throughout the entire plate thickness from the front surface through the back surface of a cutting target, depending on the absorbing characteristics of the cutting target at the wavelength of the laser-beam.
The wavelength of the laser-beam suitably selected for cutting at that time enables full-body cutting of a brittle material with a scribe depth in the direction of the entire plate thickness of the cutting target without the need of the bend-breaking step.
Further, Patent Document 2 discloses a method in which a laminated glass is irradiated with a near-infrared ray linearly to cut the laminated glass.
Patent Document 1: Japanese Patent No. 4179314
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2005-112683